Inter Gravissimas - Description


The intention expressed by the text of this bull was "to restore" the calendar so that seasonal events critical for the calculation of Easter dates would be back in their "proper places" and would be prevented from being moved away again. The idea of reform as such is not otherwise mentioned. The bull identifies "three necessary" things for the correct determination of Easter dates: correct placement of the northern vernal equinox; correct identification of the "14th day of the moon" (effectively full moon) that happens on or next after the vernal equinox, and the first Sunday that follows that full moon. The first two items were the ones that received attention; the third, about choosing the next following Sunday, was not identified as causing any problem, and was not further mentioned.

By "restore", Gregory meant two things. First, he adjusted the calendar so that the vernal equinox was near March 21, where it had been during the Council of Nicaea (May 20 – August 25, 325). This required removing ten days of drift. Second, he made the tabular 14th day of the moon correspond with the real full moon, removing "four days and more" of drift. This would restore the dates of Easter to near where they were at the time of the Council of Nicaea, although that council had not specified where in the calendar the vernal equinox should fall, and had not adopted any particular type of lunar tables. The practices of the Roman Catholic Church that had become traditional by 1582 for calculating the Easter and lunar calendars became settled when Dionysius Exiguus translated the rules of the Church of Alexandria from Greek into Latin in 525. (Britain adopted them at the Council of Whitby in 664 and France adopted them about 775. Before these years, France and Rome used the tables of Victorius of Aquitaine, which were published in 457. Britain before 664 and Rome before 457 used an 84-year Paschal cycle.)

Gregory also made changes to the calendar rules, intending to ensure that, in the future, the equinox and the 14th day of the Paschal moon, and consequently Easter Sunday, would not move away again from what the bull called their proper places.

The changes (relative to the Julian calendar) were as follows:

  1. Reduction of the number of leap years - centennial years, such as 1700, 1800, and 1900 ceased to be leap years, but years that can be divided by 400, such as 1600 and 2000 continued to be;
  2. Turning back extra days - October 4, 1582, was to be followed by October 15, 1582, and these 10 missing days were not to be counted in calculating end days of loans, taxes etc.;
  3. Easter was to be computed with reference not only to the new March 21, but also by the use of new Paschal tables.

The name of the bull consists of the first two words of the bull, which starts: "Inter gravissimas pastoralis officii nostri curas…" ("Among the most serious duties of our pastoral office…").

The bull refers to "the explanation of our calendar" and to a canon related to the dominical letter. To accompany the bull there were six chapters of explanatory rules ('canons'), and some of these (canons 1, 2, 4) refer to a book entitled Liber novæ rationis restituendi calendarii Romani (not extant) for a fuller explanation of the tables than that contained in the canons (or the bull). Because the bull, canons, and book all refer to each other, they must have been written at roughly the same time, printed at the same time (March 1), and distributed to the several countries together.

These canons enabled the computation of Easter dates in the reformed ('restored') Gregorian calendar, and gave two calendar-listings saints' days, one for the 'year of correction' (1582) and another for the entire new Gregorian year. The bull, canons, and calendars were reprinted as part of the principal book explaining and defending the Gregorian calendar, Christoph Clavius, Romani calendarii a Gregorio XIII. P. M. restituti explicatio (1603), which is tome V in his collected works Opera Mathematica (1612).

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